HUNTSVILLE, Alabama -- Federal regulators have cited TVA for a significant safety violation last fall at the Browns Ferry nuclear plant near Athens.

TVA failed to detect a faulty valve in the cooling system on the Unit 1 reactor, said the Nuclear Regulatory Commission. It gave TVA a "red" rating for the failure, which reflects "high significance" for safety.

The NRC said the valve ensures the reactor cooling system will operate properly if the plant is dealing with a fire. The failure was detected during a routine shutdown in October. The part failed sometime after it was installed in March 2009, but that failure was not detected until last fall, the NRC said. The agency faulted TVA for not having an adequate equipment testing program.

"If there is a fire, they remove electrical power from some plant components to prevent erratic behavior that could impact equipment," NRC spokesman Joey Ledford of Atlanta said Tuesday. "Because that effectively disables entire systems, a single failure in one system, such as this valve, means that system could not be counted on for cooling the core, and could potentially lead to core damage."

"TVA found the problem and fixed it," TVA's Chief Nuclear Officer Preston Swafford said in a statement.

The utility contends that the valve's failure was due to a manufacturing flaw in the size of threads in a bolt that failed.

TVA also tested the valve in a lab, Golden said, and found that the disc of the valve that stuck would eventually move due to water pressure. Golden said that means the valve would have opened between two to seven minutes after operators opened it.

The NRC disagreed with both contentions. The red status will mean additional inspections and increased regulatory scrutiny at the plant.

The NRC has issued only five red findings since its current oversight program started in 2001, according to the Associated Press.

All three units at Browns Ferry are still shut down after the April 27 storms knocked out large transmission lines that send electricity from the plant into TVA's network. Golden said it would be a week or more before the repair work on the transmission system is sufficient to restart the plant's three reactors.

Browns Ferry's reactors were knocked off-line by the storms after electricity service into the plant was disrupted. The loss of power meant the reactors went into an automatic shutdown. Because it took several days for power to be restored to the plant, TVA used diesel generators to power cooling and other vital systems at the plant.

Late on April 28, one of seven generators began leaking oil. The result was that the Unit 2 reactor was without electrical power for 28 minutes and Unit 1 was without power for about five minutes.

The NRC is continuing to look at the issues the plant faced as a result of the storm, Ledford said.